1. Field of the Invention
The field of the present invention relates generally to scrubber and filter systems used to remove or reduce to acceptable levels unwanted contaminants that are present in gas emissions. More specifically, the present invention relates to apparatuses and processes for removing contaminants from a flowing gas stream as part of an air pollution control system to remove, or reduce the level of concentration of, regulated chemicals present in the flowing gases, such as effluent gases emitted from an incinerator used to incinerate waste materials.
2. Background
As is known in the art, there are many devices and processes for removing unwanted materials, such as pollutants and other contaminants, from a gas flow stream. Most of these devices and processes are designed to remove specific chemicals or families of chemicals that are either naturally part of a flowing gas stream or which arise as by-products from the treatment of other materials, such as the effluent gases that result from the incineration of waste materials in an incinerator. The devices and processes in use primarily rely on the adsorption of the contaminant by an adsorbing material or by the physical separation of the contaminant from the gas by a filter. The goal of each of the known devices and processes is to remove the unwanted contaminants, which are usually regulated by one or more governmental bodies, or reduce the concentration of the contaminants in the flowing gas to a level that is acceptable for discharge directly to the environment.
One common use of devices and processes for the removal or reduction of contaminants from a flowing gas stream is in the field of effluent gas treatment, such as the treatment of effluent gases discharged from an incinerator used to incinerate waste materials. A difficultly for the various industries that rely on the incineration of wastes and the discharge of effluent gases to the atmosphere is that the governmental bodies and agencies responsible for the administration of such discharges (i.e., those having air pollution control responsibilities) are lowering the various acceptable levels of contaminants that are allowed in the gas that is discharged to the atmosphere. Although the reduced levels of contaminants allowed in discharged gases has as its goal an effort to improve overall air quality and reduce contamination of land and water, the result is that it makes it much more difficult and expensive for existing or new incineration facilities to treat the gas effluent from the incinerator.
One industry that has been specifically targeted by governmental agencies, particularly the United States Environmental Protection Agency (the “EPA”) with regard to the incineration of wastes is the health care industry. As is commonly known, health care facilities, including hospitals, medical treatment centers and the like, generate a significant amount of waste materials. These materials include all types of infectious waste, human pathological waste, human blood and blood products, medical equipment and other materials contaminated with human blood, sharps, animal waste exposed to infectious agents, off-spec pharmaceuticals, chemotherapeutic drug waste and low-level radioactive wastes (collectively referred to as medical waste). The majority of the medical waste is combustible materials, such as paper, animal bedding, cardboard and plastic from containers and contaminated materials. Because of the infectious nature of the medical waste material, the waste often cannot be safely or legally discharged to surface disposal facilities, such as landfills. As a result, incineration is one of the primary methods for health care facilities to dispose of their medical waste. However, due to the fact that most hospitals and other such institutions are found in highly populated urban centers, there is justified concern regarding the level of contaminants in the gases discharged to the atmosphere from the incinerator stacks.
The EPA is the primary federal agency responsible for the regulation of gas effluent discharged from health care facility incinerators. Recently, the EPA has promulgated new regulations in response to the 1990 amendments to the Clean Air Act which lower the acceptable level of certain contaminants commonly found in gaseous effluent from medical facilities. These regulations, referred to as the New Source Performance Standards (“NSPS”) for Hospital/Medical/Infectious Waste Incinerators (“HMIWI”), found at Subpart Ec of 40 CFR Part 60, enacted on Sep. 15, 1997, are applicable to health care facility incinerators constructed after Jun. 20, 1996. On Sep. 15, 1997, the EPA also promulgated Emission Guidelines for HMIWI facilities constructed on or before Jun. 20, 1996. This new rule is found at Subpart Ce of 40 CFR Part 60. Basically, the EPA rules for HMIWI facilities control the emission levels of nine regulated contaminants, namely particulate matter (PM), carbon monoxide (CO), dioxins and dibenzofurans (dioxins/furans), hydrogen chloride (HCl), sulfur dioxide (SO2), oxides of nitrogen (NOx), lead (Pb), cadmium (Cd), and mercury (Hg). Some of these contaminants, particularly the dioxins/furans, are considered carcinogens and at sufficient concentrations can cause toxic effects. The EPA regulations also place limits on the stack effluent opacity and the discharge of fugitive ash emissions, as well as requirements for operator training and qualifications, waste management plans and the testing and monitoring of pollutants. The standards for new sources are effective as of Mar. 16, 1998, and the emission guidelines for existing sources are effective as of Nov. 14, 1997. The EPA standards provide emission limits that are segregated by waste burning capacity (based on the number of pounds per hour) of the HMIWI facility. For any of the facilities, the cost of complying with the new EPA regulations is significant.
Since the regulated contaminates are both particulate (aggolmerates of molecules) and gaseous (individual gas molecules), the two most common systems for reducing all of them in the gas effluent discharge from incinerators used at HMIWI facilities are the dry scrubber or the wet scrubber, or combinations of the two. Dry scrubbers can comprise a system of contacting the flowing gas stream with a solid material which chemically reacts with the contaminants in the gas stream to form a compound. A common form of dry scrubbing is the baghouse system, which uses filter bags placed across the gas stream flow path inside a vessel, which operates under principles similar to a vacuum cleaner. The bags comprise porous materials such as woven or nonwoven fabrics, cellulose-based materials such as paper, synthetics, or other materials. The bags are configured to maximize their exposure to the gaseous flow stream. The bags may be coated with powdered carbon and powdered lime or sodium bicarbonate, by dispersing these materials in the flowing gas stream. Wet air scrubbers typically utilize one or more spray nozzles to spray water, neutralizing chemicals (such as an alkali solution) and/or other reactive chemicals into the gas flow stream and then may impart a swirling or coalescing action to the air stream in order to drop the liquid out after it has absorbed contaminants. Wet scrubbers normally evaporate water into the hot gases from the incinerator, which can be as high as 1400° to 2400° F., to reduce it to temperatures of 80° to 180° F., depending on the temperature and water content of the hot incinerator gas and specific scrubber design. The term “quenching” refers to this type of cooling. Many types of wet scrubbers are available, including venturi scrubbers, cyclonic scrubbers, spray-type cyclonic scrubbers, rotary atomizers, packed towers and others to remove particulates and gaseous impurities. Wet scrubbers have seen especially wide use due to their ability to remove both very fine particulates and toxic gases from the airstream discharged from a variety of sources, including infectious waste incinerators such as those used in hospitals. In addition, well designed wet scrubber systems operate at a lower temperature (i.e., 80° to 180° F.) than dry scrubber systems (i.e., 350° to 600° F.) and, therefore, condense pollutants such as organics more readily than dry systems. To achieve both benefits, some contaminant removal systems utilize a dry baghouse scrubber followed by a wet scrubber, such as a packed tower type wet scrubber to remove gaseous contaminants not removed by the baghouse. After removing the contaminants from the gas stream, both dry and wet scrubbers typically discharge the relatively clean gas to the atmosphere.
There are many other devices intended for scrubbing gaseous effluents in industry that work either mainly by molecular adsorption of the polluting substances on an adsorbing material, or by separation of particles through a filter. In certain types of devices, such as those using activated carbon, the adsorptive removal of contaminants works according to the principle of cohesive force and retention by chemisorption. However, none of these devices both adjust the humidity of the gases for the most effective adsorption by the chosen adsorbent and also capture entrained particulate dust before final discharge of the cleaned gases.
Although the presently known and available contaminant removal systems are capable of removing the EPA regulated pollutants, they may not be able to economically achieve the lower levels now required for HMIWI, particularly with regard to the maximum levels allowed for mercury, lead and dioxins/furans, and at the same time meeting the HCl emission requirements. It can be appreciated, therefore, that what is needed is an apparatus and process for efficiently removing contaminants, including the contaminants described above, from effluent stream of gas, particularly a flowing gas stream resulting from the incineration of medical and related wastes. Such an apparatus and process should be able to effectively and efficiently remove or reduce to below acceptable standards the level of all contaminants in the effluent stream.